1. Field of the Invention
The present invention relates to a waveguide type semiconductor photodetecting device, in which an optical waveguide and a semiconductor photodetector optically coupled with the optical waveguide are integrated on a semiconductor substrate.
2. Description of the Related Art
A semiconductor photodetecting device employing a compound semiconductor has been widely employed as an element for optical communication, since such semiconductor photodetecting device facilitates obtaining of an element having a wavelength of 1.3 .mu.m to 1.5 .mu.m which is a low loss region of an optical fiber. As one example of the photodetecting element for optical communication can be InGaAs p-i-n photodiode, for which demands for speeding of response characteristics and enhancement of reception sensitivity have been present.
In order to improve photoelectric conversion efficiency required for enhancement of the reception sensitivity, when a light incides perpendicularly to a light absorption layer, higher efficiency can be obtained by providing thicker InGaAs light absorption layer. However, when the InGaAs light absorption layer becomes excessively thick, degradation of response speed can be caused due to limitation of traveling period of carrier. Thus, thickness of the InGaAs light absorption layer should be limited. In order to solve such trade off, attention has been paid for a waveguide type photodetecting device, in which the waveguide and the photodetecting device are integrated, as element structure. In this photodetecting device, high speed and high sensitivity element characteristics can be obtained by inciding an incident light in a direction parallel to the light absorption layer.
On the other hand, the p-i-n photodiode with the waveguide encounters a problem in low coupling tolerance with a fiber input. Namely, the p-i-n photodiode with the waveguide having an approximately 50 GHz band has inciding end face of 5 .mu.m of waveguide width and 0.5 .mu.m of light absorption layer thickness, which is significantly smaller than the typical surface incident type p-i-n photodiode. Then, slight offset of the inciding end face from focal position of the incident light should cause significant fluctuation of sensitivity.
In the p-i-n photodiode with the waveguide having a band in excess of 50 GHz, it becomes necessary to narrow the waveguide width to be less than or equal to 2 .mu.m for lowering element capacity. In such case, sensitivity should be lowered for lowering of the coupling efficiency per se even when no offset is present between the end face and the focal position.
As prior art solving the problem in lowering of coupling tolerance and coupling efficiency in the p-i-n photodiode with the waveguide, a waveguide formed into tapered shape (hereinafter referred to as "tapered waveguide") is integrated at the incident side of the element to converting the spot size of the incident light to introduce into the element. One example of the conventional integration circuit "IEICE TRANS. ELECTRON., VOL. E-76-C, No. 2, p. 214, 1993) is shown in FIGS. 1A and 1B.
FIG. 1A is a plan view and FIG. 1B is a section view of the prior art. In this prior art, on a semi-insulative InP substrate 21, an InP buffer layer 22, a p.sup.+ -InGaAsP contact layer 23, an n.sup.- -InGaAs light absorption layer 24 are provided. With partly overlapping with a stack of the foregoing layers, a tapered waveguide consisted of an n.sup.+ -InP clad layer 25, an n.sup.+ -InGaAsP core layer 26 and an n.sup.+ -InP clad layer 27 is provided. Furthermore, a p-side electrode 28 is provided on the p.sup.+ -InGaAs contact layer 23, and n-side electrode 29 is provided on the n.sup.+ -InP clad layer 27. Then, a width of the tapered waveguide is set to be 1 to 2 .mu.m matching with the p-i-n photodiode at the p-i-n photodiode side and is gradually widened to be 4 .mu.m at the inciding end face for improving coupling efficiency of the incident light.
In the conventional tapered waveguide such as that illustrated in FIGS. 1A and 1B, when the width of the waveguide at the inciding end face side is widened beyond a certain extent, higher-order mode may newly appear as the waveguiding mode in horizontal direction. On the other hand, since the width of the waveguide is narrowed at the p-i-n photodiode side to cut off such higher-order mode. Thus, all of the higher-order mode light excited at the inciding end face is reflected at the midway in propagation through the tapered waveguide to be a loss (see FIGS. 9A-9D). Accordingly, the width of the waveguide at the inciding end face can be widened only within a range where the problem set forth above will not be caused. Therefore, it is not always possible to obtain sufficient coupling tolerance even in the solution as illustrated in FIGS. 1A and 1B.
On the other hand, in the shown prior art, the core layer of the tapered waveguide and the core layer (light absorption layer) of the p-i-n photodiode are respectively formed by at least twice of crystal growth, and an etching step for the crystal layer is present between the crystal growth steps, it is difficult to realize high coupling efficiency between the tapered waveguide and the p-i-n photodiode at high yield.